Method and apparatus for providing channel state information-reference signal (CSI-RS) configuration information in a wireless communication system supporting multiple antennas

ABSTRACT

A method and a mobile station for transmitting channel state information (CSI) to a base station; and a method and a base station for receiving CSI from a mobile station in a wireless communication system are discussed. The method according to an embodiment includes receiving first information on one or more channel quality measurement resources and second information on one or more interference measurement resources from a base station; receiving reference signals based on the first information from the base station; generating the CSI by using the reference signals and the second information; and transmitting the CSI to the base station.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.14/961,612 filed on Dec. 7, 2015, which is a Continuation of U.S. patentapplication Ser. No. 14/622,332 filed on Feb. 13, 2015 (now U.S. Pat.No. 9,236,990 issued on Jan. 12, 2016), which is a Continuation of U.S.patent application Ser. No. 13/510,222 filed on May 16, 2012 (now U.S.Pat. No. 8,989,114 issued on Mar. 24, 2015), which is filed as theNational Phase of PCT/KR2011/001833 filed on Mar. 16, 2011, which claimsthe benefit under 35 U.S.C. § 119(e) to U.S. Provisional Application No.61/413,924 filed on Nov. 15, 2010 and 61/314,981 filed on Mar. 17, 2010,all of which are hereby expressly incorporated by reference into thepresent application.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a wireless communication system, andmore particularly, to a method and an apparatus for providing ChannelState Information-Reference Signal (CSI-RS) configuration information ina wireless communication system supporting multiple antennas.

Discussion of the Related Art

A Multiple Input Multiple Output (MIMO) system refers to a system forimproving data transmission/reception efficiency using multipleTransmission (Tx) antennas and multiple Reception (Rx) antennas. In aMIMO system, each transmission antenna has an independent data channel.The Tx may be a virtual antenna or a physical antenna. A receiverestimates a channel with respect to each Tx antenna and receives datatransmitted from each Tx antenna based on the channel estimation.

Channel estimation refers to a process of compensating for signaldistortion caused by fading so as to restore the received signal. Fadingrefers to a phenomenon in which the intensity of a signal is rapidlychanged due to multi-path time delay in a wireless communication systemenvironment. For channel estimation, a reference signal known to both atransmitter and a receiver is necessary. The reference signal may beabbreviated to RS or referred to as a pilot signal according to thestandard.

A downlink RS is a pilot signal for coherent demodulation, such as aPhysical Downlink Shared Channel (PDSCH), a Physical Control FormatIndicator Channel (PCFICH), a Physical Hybrid Indicator Channel (PHICH),and a Physical Downlink Control Channel (PDCCH). The downlink RSincludes a Common Reference Signal (CRS) shared among all UEs in a celland a Dedicated Reference Signal (DRS) for a specific UE.

In a system having an antenna configuration (e.g., a system according tothe Long Term Evolution-Advanced (LTE-A) standard supporting eight Txantennas) developed as an extension of a legacy communication system(e.g., a system based on LTE Release 8 or 9) supporting four Txantennas, DRS-based data demodulation has been considered in order tosupport efficient RS management and develop an advanced transmissionscheme. That is, in order to support data transmission through extendedantennas, DRSs for two or more layers may be defined. Since the DRSs areprecoded using the same precoder as used for data, it is possible for areceiver to easily estimate channel information for demodulating datawithout separate precoding information.

A downlink receiver may acquire precoded channel information withrespect to the extended antenna configuration through DRSs. However, inorder to acquire non-precoded channel information, separate RSs arerequired in addition to the DRSs. In a system based on the LTE-Astandard, RSs for acquiring Channel State Information (CSI) at areceiver, that is, CSI-RSs, may be defined.

SUMMARY OF THE INVENTION

An object of the present invention devised to solve the problem lies onefficient and accurate measurement and reporting of Channel StateInformation (CSI) using one or more Channel State Information-ReferenceSignal (CSI-RS) configurations.

The object of the present invention can be achieved by providing amethod for transmitting Channel State Information-Reference Signals(CSI-RSs) from a base station supporting multiple transmit antennas to amobile station, including transmitting, at the base station, informationof one or more CSI-RS configurations to the mobile station, wherein theone or more CSI-RS configurations include one CSI-RS configuration forwhich the mobile station assumes non-zero transmission power for CSI-RS,transmitting, at the base station, information indicating CSI-RSconfiguration for which the mobile station assumes zero transmissionpower for CSI-RS among the one or more CSI-RS configurations to themobile station, mapping, at the base station, CSI-RSs to resourceelements of a downlink subframe based on the one or more CSI-RSconfigurations, and transmitting, at the base station, the downlinksubframe mapped with the CSI-RSs to the mobile station.

In another aspect of the present invention, provided herein is a methodfor transmitting CSI at a mobile station using a CSI-RS from a basestation supporting multiple transmit antennas, including receiving, atthe mobile station, information of one or more CSI-RS configurationsfrom the base station, wherein the one or more CSI-RS configurationsinclude one CSI-RS configuration for which the mobile station assumesnon-zero transmission power for a CSI-RS, receiving, at the mobilestation, information indicating a CSI-RS configuration for which themobile station assumes zero transmission power for a CSI-RS among theone or more CSI-RS configurations from the base station, receiving, atthe mobile station, a downlink subframe of which resource elements aremapped with CSI-RSs based on the one or more CSI-RS configurations fromthe base station, and transmitting, at the mobile station, the CSImeasured by using the CSI-RSs to the base station.

In another aspect of the present invention, provided herein is a basestation for transmitting a CSI-RS for multiple antennas transmission,including a receiving module for receiving an uplink signal from amobile station, a transmitting module for transmitting a downlink signalto the mobile station, and a processor for controlling the base stationcomprising the receiving module and the transmitting module. Theprocessor is configured to transmit, via the transmitting module,information of one or more CSI-RS configurations to the mobile station,the one or more CSI-RS configurations including one CSI-RS configurationfor which the mobile station assumes non-zero transmission power for aCSI-RS, transmit, via the transmitting module, information indicating aCSI-RS configuration for which the mobile station assumes zerotransmission power for a CSI-RS among the one or more CSI-RSconfigurations to the mobile station, map CSI-RSs to resource elementsof a downlink subframe based on the one or more CSI-RS configurations,and transmit, via the transmitting module, the downlink subframe mappedwith the CSI-RSs to the mobile station.

In a further aspect of the present invention, provided herein is amobile station for transmitting CSI using a CSI-RS from a base stationsupporting multiple transmit antennas, including a receiving module forreceiving a downlink signal from the base station, a transmitting modulefor transmitting an uplink signal to the base station, and a processorfor controlling the mobile station comprising the receiving module andthe transmitting module. The processor is configured to receive, via thereceiving module, information of one or more CSI-RS configurations fromthe base station, wherein the one or more CSI-RS configurations includeone CSI-RS configuration for which the mobile station assumes non-zerotransmission power for a CSI-RS, receive, via the receiving module,information indicating a CSI-RS configuration for which the mobilestation assumes zero transmission power for a CSI-RS among the one ormore CSI-RS configurations from the base station, receive, via thereceiving module, a downlink subframe of which resource elements aremapped with CSI-RSs based on the one or more CSI-RS configurations fromthe base station, and transmit, via the transmitting module, the CSImeasured by using the CSI-RSs to the base station.

In each aspect of the present invention, the one or more CSI-RSconfigurations may indicate positions of the resource elements mappedwith the CSI-RSs.

The downlink subframe mapped with the CSI-RSs may be configured by apredetermined period and a predetermined offset.

The predetermined period and the predetermined offset may be configuredas cell-specific.

The predetermined period and the predetermined offset may be configuredseparately for CSI-RSs for which the mobile station assume non-zero andzero transmission power.

The CSI-RS configuration for which the mobile station assumes zerotransmission power for the CSI-RS may correspond to positions ofresource elements where CSI-RSs of neighbor base station aretransmitted.

The CSI-RSs may be transmitted for one, two, four or eight antennaports.

The BS may transmit to the mobile station an indication of a CSI-RSconfiguration used for CSI feedback by the mobile station among the oneor more CSI-RS configurations, through dedicated RRC (Radio ResourceControl) signaling.

The above-mentioned general description of the present invention and thefollowing detailed description of the present invention are merelyexemplary and provide an additional description of the appended claimsof the present invention.

According to embodiments of the present invention, CSI can beefficiently and accurately measured and reported using one or moreCSI-RS configurations.

Additional advantages of the present invention will be set forth in partin the description which follows and in part will become apparent tothose having ordinary skill in the art upon examination of the followingor may be learned from practice of the present application.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a diagram illustrating the structure of a downlink radioframe.

FIG. 2 is a diagram illustrating an example of a resource grid for theduration of one downlink slot.

FIG. 3 is a diagram illustrating the structure of a downlink subframe.

FIG. 4 is a diagram illustrating the structure of an uplink subframe.

FIG. 5 is a diagram illustrating the configuration of a wirelesscommunication system having multiple antennas.

FIG. 6 illustrates a conventional Common Reference Signal (CRS) andDedicated Reference Signal (DRS) pattern.

FIG. 7 illustrates an exemplary Demodulation Reference Signal (DM RS)pattern.

FIG. 8 illustrates exemplary Channel State Information-Reference Signal(CSI-RS) patterns.

FIG. 9 is a diagram referred to for describing an exemplary periodicCSI-RS transmission.

FIG. 10 is a diagram referred to for describing an exemplary aperiodicCSI-RS transmission.

FIG. 11 is a diagram referred to for describing an example of using twoCSI-RS configurations.

FIG. 12 is a diagram referred to for describing mapping between CSI-RSsand Resource Elements (REs) according to the number of antennas.

FIG. 13 is a diagram illustrating a signal flow for a method fortransmitting CSI-RS configuration information according to an embodimentof the present invention.

FIG. 14 is a block diagram of an evolved Node B (eNB) apparatus and aUser Equipment (UE) apparatus according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following embodiments are proposed by combining constituentcomponents and characteristics of the present invention according to apredetermined format. The individual constituent components orcharacteristics should be considered to be optional factors on thecondition that there is no additional remark. If required, theindividual constituent components or characteristics may not be combinedwith other components or characteristics. Also, some constituentcomponents and/or characteristics may be combined to implement theembodiments of the present invention. The order of operations to bedisclosed in the embodiments of the present invention may be changed toanother. Some components or characteristics of any embodiment may alsobe included in other embodiments, or may be replaced with those of theother embodiments as necessary.

The embodiments of the present invention are disclosed on the basis of adata communication relationship between a Base Station (BS) and aterminal. In this case, the BS is used as a terminal node of a networkvia which the BS can directly communicate with the terminal. Specificoperations to be conducted by the BS in the present invention may alsobe conducted by an upper node of the BS as necessary.

In other words, it will be obvious to those skilled in the art thatvarious operations for enabling the BS to communicate with the terminalin a network composed of several network nodes including the BS will beconducted by the BS or other network nodes other than the BS. The term“BS” may be replaced with a fixed station, Node B, evolved Node B (eNBor eNode B), or an Access Point (AP) as necessary. The term “relay” maybe replaced with a Relay Node (RN) or a Relay Station (RS). The term“terminal” may also be replaced with a User Equipment (UE), a MobileStation (MS), a Mobile Subscriber Station (MSS) or a Subscriber Station(SS) as necessary.

It should be noted that specific terms disclosed in the presentinvention are proposed for the convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to another format within the technical scope orspirit of the present invention.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention and theimportant functions of the structures and devices are shown in blockdiagram form. The same reference numbers will be used throughout thedrawings to refer to the same or like parts.

Embodiments of the present invention are supported by standard documentsdisclosed for at least one of wireless access systems including anInstitute of Electrical and Electronics Engineers (IEEE) 802 system, a3rd Generation Project Partnership (3GPP) system, a 3GPP Long TermEvolution (LTE) system, and a 3GPP2 system. In particular, the steps orparts, which are not described to clearly reveal the technical idea ofthe present invention, in the embodiments of the present invention maybe supported by the above documents. All terminology used herein may besupported by at least one of the above-mentioned documents.

The following embodiments of the present invention can be applied to avariety of wireless access technologies, for example, Code DivisionMultiple Access (CDMA), Frequency Division Multiple Access (FDMA), TimeDivision Multiple Access (TDMA), Orthogonal Frequency Division MultipleAccess (OFDMA), Single Carrier Frequency Division Multiple Access(SC-FDMA), and the like. CDMA may be embodied with wireless (or radio)technology such as Universal Terrestrial Radio Access (UTRA) orCDMA2000. TDMA may be embodied with wireless (or radio) technology suchas Global System for Mobile communications (GSM)/General Packet RadioService (GPRS)/Enhanced Data Rates for GSM Evolution (EDGE). OFDMA maybe embodied with wireless (or radio) technology such as Institute ofElectrical and Electronics Engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16(WiMAX), IEEE 802-20, and Evolved UTRA (E-UTRA). UTRA is a part ofUniversal Mobile Telecommunications System (UMTS). 3rd GenerationPartnership Project Long Term Evolution (3GPP LTE) is a part of EvolvedUMTS (E-UMTS), which uses E-UTRA. 3GPP LTE employs OFDMA in downlink andemploys SC-FDMA in uplink. LTE-Advanced (LTE-A) is an evolution of 3GPPLTE. WiMAX can be explained by an IEEE 802.16e (WirelessMAN-OFDMAReference System) and an advanced IEEE 802.16m (WirelessMAN-OFDMAAdvanced System). For clarity, the following description focuses on the3GPP LTE and LTE-A systems. However, the technical features of thepresent invention are not limited thereto.

The structure of a downlink radio frame will be described with referenceto FIG. 1.

In a cellular Orthogonal Frequency Division Multiplexing (OFDM) radiopacket communication system, uplink/downlink data packets aretransmitted in subframes. One subframe is defined as a predeterminedtime interval including a plurality of OFDM symbols. The 3GPP LTEstandard supports a type 1 radio frame structure applicable to FrequencyDivision Duplex (FDD) and a type 2 radio frame structure applicable toTime Division Duplex (TDD).

FIG. 1 is a diagram illustrating the structure of the type 1 radioframe. A downlink radio frame includes 10 subframes, and one subframeincludes two slots in the time domain. A time required for transmittingone subframe is defined as a Transmission Time Interval (TTI). Forexample, one subframe may have a length of 1 ms and one slot may have alength of 0.5 ms. One slot may include a plurality of OFDM symbols inthe time domain and include a plurality of Resource Blocks (RBs) in thefrequency domain. Since the 3GPP LTE system uses OFDMA in downlink, theOFDM symbol indicates one symbol duration. The OFDM symbol may be calledan SC-FDMA symbol or symbol duration. An RB is a resource allocationunit including a plurality of contiguous subcarriers in one slot.

The number of OFDM symbols included in one slot may be changed accordingto the configuration of a Cyclic Prefix (CP). There are an extended CPand a normal CP. For example, the number of OFDM symbols included in oneslot may be seven in case of a normal CP. In case of an extended CP, thelength of one OFDM symbol is increased and thus the number of OFDMsymbols included in one slot is less than that in case of a normal CP.In case of the extended CP, for example, the number of OFDM symbolsincluded in one slot may be six. If a channel state is instable as isthe case when a UE moves fast, the extended CP may be used in order tofurther reduce interference between symbols.

In case of a normal CP, since one slot includes seven OFDM symbols, onesubframe includes 14 OFDM symbols. The first two or three OFDM symbolsof each subframe may be allocated to a Physical Downlink Control Channel(PDCCH) and the remaining OFDM symbols may be allocated to a PhysicalDownlink Shared Channel (PDSCH).

The structure of the radio frame is only exemplary. Accordingly, thenumber of subframes included in a radio frame, the number of slotsincluded in a subframe or the number of symbols included in a slot maybe changed in various manners.

FIG. 2 is a diagram illustrating an example of a resource grid in onedownlink slot. OFDM symbols are configured by the normal CP. Referringto FIG. 2, the downlink slot includes a plurality of OFDM symbols in thetime domain and includes a plurality of RBs in the frequency domain.Although FIG. 2 exemplarily depicts that one downlink slot includesseven OFDM symbols and one RB includes 12 subcarriers, the presentinvention is not limited thereto. Each element of the resource grid isreferred to as a Resource Element (RE). For example, an RE a(k,l) islocated at a kth subcarrier and an lth OFDM symbol. In case of a normalCP, one RB includes 12×7 REs (in case of an extended CP, one RB includes12×6 REs). Since the spacing between subcarriers is 15 kHz, one RB isabout 180 kHz in the frequency domain. NDL denotes the number of RBsincluded in the downlink slot. NDL is determined based on a downlinktransmission bandwidth set through Node B scheduling.

FIG. 3 is a diagram illustrating the structure of a downlink subframe.Up to three OFDM symbols at the start of a first slot of one subframecorresponds to a control region to which a control channel is allocated.The remaining OFDM symbols correspond to a data region to which aPhysical Downlink Shared Channel (PDSCH) is allocated. A basictransmission unit is one subframe. That is, a PDCCH and a PDSCH areallocated across two slots. Examples of the downlink control channelsused in the 3GPP LTE system include, for example, a Physical ControlFormat Indicator Channel (PCFICH), a Physical Downlink Control Channel(PDCCH), a Physical Hybrid automatic repeat request Indicator Channel(PHICH), etc. The PCFICH is located in the first OFDM symbol of asubframe, carrying information about the number of OFDM symbols used forcontrol channels in the subframe. The PHICH includes a HARQACKnowledgment/Negative ACKnowledgment (ACK/NACK) signal as a responseto an uplink transmission. The control information transmitted on thePDCCH is referred to as Downlink Control Information (DCI). The DCIincludes uplink or downlink scheduling information or an uplink transmitpower control command for a certain UE group. The PDCCH may includeinformation about resource allocation and transmission format of aDownlink Shared Channel (DL-SCH), resource allocation information of anUplink Shared Channel (UL-SCH), paging information of a Paging Channel(PCH), system information on the DL-SCH, information about resourceallocation of an higher layer control message such as a Random AccessResponse (RAR) transmitted on the PDSCH, a set of transmit power controlcommands for individual UEs in a certain UE group, transmit powercontrol information, information about activation of Voice over IP(VoIP), etc. A plurality of PDCCHs may be transmitted in the controlregion. A UE may monitor the plurality of PDCCHs. The PDCCHs aretransmitted on an aggregation of one or several contiguous ControlChannel Elements (CCEs). A CCE is a logical allocation unit used toprovide the PDCCHs at a coding rate based on the state of a radiochannel. The CCE includes a set of REs. A format and the number ofavailable bits for the PDCCH are determined based on the correlationbetween the number of CCEs and the coding rate provided by the CCEs. TheBS determines a PDCCH format according to DCI to be transmitted to theUE, and attaches a Cyclic Redundancy Check (CRC) to control information.The CRC is masked by a Radio Network Temporary Identifier (RNTI)according to the owner or usage of the PDCCH. If the PDCCH is for aspecific UE, the CRC may be masked by a cell-RNTI (C-RNTI) of the UE. Ifthe PDCCH is for a paging message, the CRC may be masked by a pagingindicator identifier (P-RNTI). If the PDCCH is for system information(more specifically, a System Information Block (SIB)), the CRC may bemasked by a system information identifier and a System Information RNTI(SI-RNTI). To indicate a random access response to a random accesspreamble received from the UE, the CRC may be masked by a randomaccess-RNTI (RA-RNTI).

FIG. 4 is a diagram illustrating the structure of an uplink subframe.The uplink subframe may be divided into a control region and a dataregion in the frequency domain. A Physical Uplink Control Channel(PUCCH) including uplink control information is allocated to the controlregion. A Physical uplink Shared Channel (PUSCH) including user data isallocated to the data region. In order to maintain single carrierproperty, one UE does not simultaneously transmit the PUCCH and thePUSCH. A PUCCH for one UE is allocated to an RB pair in a subframe. TheRBs of the RB pair occupy different subcarriers in two slots. Thus, theRB pair allocated to the PUCCH is “frequency-hopped” over a slotboundary.

Modeling of Multiple Input Multiple Output (MIMO) System

The MIMO system increases data transmission/reception efficiency using aplurality of Tx antennas and a plurality of Rx antennas. MIMO is anapplication of putting data segments received from a plurality ofantennas into a whole message, without depending on a single antennapath to receive the whole message.

MIMO schemes are classified into spatial diversity and spatialmultiplexing. Spatial diversity increases transmission reliability or acell radius using diversity gain and thus is suitable for datatransmission for a fast moving UE. In spatial multiplexing, multiple Txantennas simultaneously transmit different data and thus high-speed datacan be transmitted without increasing a system bandwidth.

FIG. 5 illustrates the configuration of a wireless communication systemsupporting multiple antennas. Referring to FIG. 5(a), when the number ofTransmission (Tx) antennas and the number of Reception (Rx) antennas areincreased to NT and NR, respectively at both a transmitter and areceiver, a theoretical channel transmission capacity increases inproportion to the number of antennas, compared to use of a plurality ofantennas at only one of the transmitter and the receiver. Therefore,transmission rate and frequency efficiency are remarkably increased.Along with the increase of channel transmission capacity, thetransmission rate may be increased in theory to the product of a maximumtransmission rate Ro that may be achieved in case of a single antennaand a rate increase rate Ri.R _(i)=min(N _(T) ,N _(R))  [Equation 1]

For instance, a MIMO communication system with four Tx antennas and fourRx antennas may achieve a four-fold increase in transmission ratetheoretically, relative to a single-antenna wireless communicationsystem. Since the theoretical capacity increase of the MIMO wirelesscommunication system was proved in the mid 1990's, many techniques havebeen actively studied to increase data rate in real implementation. Someof the techniques have already been reflected in various wirelesscommunication standards including standards for 3G mobilecommunications, future-generation Wireless Local Area Network (WLAN),etc.

Concerning the research trend of MIMO up to now, active studies areunderway in many respects of MIMO, inclusive of studies of informationtheory related to calculation of multi-antenna communication capacity indiverse channel environments and multiple access environments, studiesof measuring MIMO radio channels and MIMO modeling, studies oftime-space signal processing techniques to increase transmissionreliability and transmission rate, etc.

Communication in a MIMO system with NT Tx antennas and NR Rx antennaswill be described in detail through mathematical modeling.

Regarding a transmission signal, up to NT pieces of information can betransmitted through the NT Tx antennas, as expressed as the followingvector.s=└s ₁ ,s ₂ , . . . ,s _(N) _(T) ┘^(T)  [Equation 2]

A different transmission power may be applied to each piece oftransmission information, s₁, s₂, . . . , s_(N) _(T) . Let the transmitpower levels of the transmission information be denoted by P₁, P₂, . . ., P_(N) _(T) , respectively. Then the transmission power-controlledtransmission information vector may be given asŝ=[ŝ ₁ ,ŝ ₂ , . . . ,ŝ _(N) _(T) ]^(T) =[P ₁ s ₁ ,P ₂ s ₂ , . . . ,P_(N) _(T) s _(N) _(T) ]^(T)  [Equation 3]

The transmission power-controlled transmission information vector s maybe expressed as follows, using a diagonal matrix P of transmissionpower.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

NT transmission signals x₁, x₂, . . . , x_(N) _(T) may be generated bymultiplying the transmission power-controlled information vector ŝ by aweight matrix W. The weight matrix W functions to appropriatelydistribute the transmission information to the Tx antennas according totransmission channel states, etc. These NT transmission signals x₁, x₂,. . . , x_(N) _(T) are represented as a vector x, which may bedetermined as

$\begin{matrix}{x = {\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{bmatrix} = {\quad{{\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1N_{T}} \\w_{21} & w_{22} & \ldots & w_{2N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 1} & w_{i\; 2} & \ldots & w_{{iN}_{T}} \\\vdots & \; & {\;\ddots} & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {{W\hat{s}} = {WPs}}}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Here, w_(ij) denotes a weight between a jth piece of information and anith Tx antenna and W is a precoding matrix.

The transmitted signal x may be differently processed using according totwo schemes (for example, spatial diversity and spatial multiplexing).In spatial multiplexing, different signals are multiplexed andtransmitted to a receiver such that elements of information vector(s)have different values. In spatial diversity, the same signal isrepeatedly transmitted through a plurality of channel paths such thatelements of information vector(s) have the same value. Spatialmultiplexing and spatial diversity may be used in combination. Forexample, the same signal may be transmitted through three Tx antennas inspatial diversity, while the remaining signals may be transmitted to thereceiver in spatial multiplexing.

Given NR Rx antennas, signals received at the Rx antennas, y₁, y₂, . . ., y_(N) _(R) may be represented as the following vector.y=[y ₁ ,y ₂ , . . . ,y _(N) _(R) ]^(T)  [Equation 6]

When channels are modeled in the MIMO wireless communication system,they may be distinguished according to the indexes of Tx and Rxantennas. A channel between a jth Tx antenna and an ith Rx antenna isdenoted by hij. Notably, the index of an Rx antenna precedes the indexof a Tx antenna in hij.

FIG. 5(b) illustrates channels from NT Tx antennas to an ith Rx antenna.The channels may be collectively represented as a vector or a matrix.Referring to FIG. 5(b), the channels from the NT Tx antennas to the ithRx antenna may be expressed as [Equation 7].h _(i) ^(T) =[h _(i1) ,h _(i2) , . . . ,h _(iN) _(T) ]  [Equation 7]

Hence, all channels from the NT Tx antennas to the NR Rx antennas may beexpressed as the following matrix.

$\begin{matrix}{H = {\begin{bmatrix}h_{1}^{T} \\h_{2}^{T} \\\vdots \\h_{i}^{T} \\\vdots \\h_{N_{R}}^{T}\end{bmatrix} = \begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{21} & h_{22} & \ldots & h_{2N_{T}} \\\vdots & \; & {\;\ddots} & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 8} \right\rbrack\end{matrix}$

Actual channels experience the above channel matrix H and then are addedwith Additive White Gaussian Noise (AWGN). The AWGN n₁, n₂, . . . ,n_(N) _(R) added to the NR Rx antennas is given as the following vector.n=[n ₁ ,n ₂ , . . . ,n _(N) _(R) ]^(T)  [Equation 9]

From the above mathematical modeling, the received signal vector isgiven as

$\begin{matrix}{y = {\begin{bmatrix}y_{1} \\y_{2} \\\vdots \\y_{i} \\\vdots \\y_{N_{R}}\end{bmatrix} = {{{\begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{21} & h_{22} & \ldots & h_{2N_{T}} \\\vdots & \; & {\;\ddots} & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{j} \\\vdots \\x_{N_{T}}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2} \\\vdots \\n_{i} \\\vdots \\n_{N_{R}}\end{bmatrix}} = {{Hx} + n}}}} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack\end{matrix}$

The numbers of rows and columns in the channel matrix H representingchannel states are determined according to the numbers of Rx and Txantennas. Specifically, the number of rows in the channel matrix H isequal to the number of Rx antennas, NR and the number of columns in thechannel matrix H is equal to the number of Tx antennas, NT. Hence, thechannel matrix H is of size NR×NT.

The rank of a matrix is defined as the smaller between the number ofindependent rows and the number of independent columns in the matrix.Accordingly, the rank of the matrix is not larger than the number ofrows or columns of the matrix. The rank of the channel matrix H, rank(H)satisfies the following constraint.rank(H)≤min(N _(T) ,N _(R))  [Equation 11]

In MIMO transmission, the term “rank” denotes the number of paths forindependently transmitting signals, and the term “number of layers”denotes the number of signal streams transmitted through respectivepaths. In general, since a transmitter transmits as many layers as thenumber of ranks used for signal transmission, the rank has the samemeaning as the number of layers unless otherwise noted.

Reference Signals (RSs)

In a wireless communication system, a packet is transmitted on a radiochannel. In view of the nature of the radio channel, the packet may bedistorted during the transmission. To receive the signal successfully, areceiver should compensate for the distortion of the received signalusing channel information. Generally, to enable the receiver to acquirethe channel information, a transmitter transmits a signal known to boththe transmitter and the receiver and the receiver acquires knowledge ofchannel information based on the distortion of the signal received onthe radio channel. This signal is called a pilot signal or an RS.

In case of data transmission and reception through multiple antennas,knowledge of channel states between Tx antennas and Rx antennas isrequired for successful signal reception. Accordingly, an RS shouldexist for each Tx antenna.

In a mobile communication system, RSs are largely categorized into twotypes according to the purposes that they serve, RSs used foracquisition of channel information and RSs used for data demodulation.The former-type RSs should be transmitted in a wideband to enable UEs toacquire downlink channel information. Even UEs that do not receivedownlink data in a specific subframe should be able to receive such RSsand measure them. When an eNB transmits downlink data, it transmits thelatter-type RSs in resources allocated to the downlink data. A UE canperform channel estimation by receiving the RSs and thus demodulate databased on the channel estimation. These RSs should be transmitted in adata transmission region.

In the legacy 3GPP LTE system (e.g. one conforming to 3GPP LTERelease-8), two types of downlink RSs are defined for unicast service,Common RS (CRS) and Dedicated RS (DRS). CRS is used for CSI acquisitionand measurement, for example, for handover. The CRS is also called acell-specific RS. DRS is used for data demodulation, called aUE-specific RS. The legacy 3GPP LTE system uses the DRS only for datademodulation and the CRS for the two purposes of channel informationacquisition and data demodulation.

CRSs, which are cell-specific, are transmitted across a wideband inevery subframe. According to the number of Tx antennas at an eNB, theeNB may transmit CRSs for up to four antenna ports. For instance, an eNBwith two Tx antennas transmits CRSs for antenna port 0 and antennaport 1. If the eNB has four Tx antennas, it transmits CRSs forrespective four Tx antenna ports, antenna port 0 to antenna port 3.

FIG. 6 illustrates a CRS and DRS pattern for an RB (including 14 OFDMsymbols in time by 12 subcarriers in frequency in case of a normal CP)in a system where an eNB has four Tx antennas. In FIG. 6, REs labeledwith ‘R0’, ‘R1’, ‘R2’ and ‘R3’ represent the positions of CRSs forantenna port 0 to antenna port 4, respectively. REs labeled with ‘D’represent the positions of DRSs defined in the LTE system.

The LTE-A system, an evolution of the LTE system, can support up toeight Tx antennas. Therefore, it should also support RSs for up to eightTx antennas. Because downlink RSs are defined only for up to four Txantennas in the LTE system, RSs should be additionally defined for fiveto eight Tx antenna ports, when an eNB has five to eight downlink Txantennas in the LTE-A system. Both RSs for channel measurement and RSsfor data demodulation should be considered for up to eight Tx antennaports.

One of significant considerations for design of the LTE-A system isbackward compatibility. Backward compatibility is a feature thatguarantees a legacy LTE terminal to operate normally even in the LTE-Asystem. If RSs for up to eight Tx antenna ports are added to atime-frequency area in which CRSs defined by the LTE standard aretransmitted across a total frequency band in every subframe, RS overheadbecomes huge. Therefore, new RSs should be designed for up to eightantenna ports in such a manner that RS overhead is reduced.

Largely, new two types of RSs are introduced to the LTE-A system. Onetype is CSI-RS serving the purpose of channel measurement for selectionof a transmission rank, a Modulation and Coding Scheme (MCS), aPrecoding Matrix Index (PMI), etc. The other type is Demodulation RS (DMRS) for demodulation of data transmitted through up to eight Txantennas.

Compared to the CRS used for both purposes of measurement such aschannel measurement and measurement for handover and data demodulationin the legacy LTE system, the CSI-RS is designed mainly for channelestimation, although it may also be used for measurement for handover.Since CSI-RSs are transmitted only for the purpose of acquisition ofchannel information, they may not be transmitted in every subframe,unlike CRSs in the legacy LTE system. Accordingly, CSI-RSs may beconfigured so as to be transmitted intermittently (e.g. periodically)along the time axis, for reduction of CSI-RS overhead.

When data is transmitted in a downlink subframe, DM RSs are alsotransmitted dedicatedly to a UE for which the data transmission isscheduled. Thus, DM RSs dedicated to a particular UE may be designedsuch that they are transmitted only in a resource area scheduled for theparticular UE, that is, only in a time-frequency area carrying data forthe particular UE.

FIG. 7 illustrates an exemplary DM RS pattern defined for the LTE-Asystem. In FIG. 7, the positions of REs carrying DM RSs in an RBcarrying downlink data (an RB having 14 OFDM symbols in time by 12subcarriers in frequency in case of a normal CP) are marked. DM RSs maybe transmitted for additionally defined four antenna ports, antenna port7 to antenna port 10 in the LTE-A system. DM RSs for different antennaports may be identified by their different frequency resources(subcarriers) and/or different time resources (OFDM symbols). This meansthat the DM RSs may be multiplexed in Frequency Division Multiplexing(FDM) and/or Time Division Multiplexing (TDM). If DM RSs for differentantenna ports are positioned in the same time-frequency resources, theymay be identified by their different orthogonal codes. That is, these DMRSs may be multiplexed in Code Division Multiplexing (CDM). In theillustrated case of FIG. 7, DM RSs for antenna port 7 and antenna port 8may be located on REs of DM RS CDM group 1 through multiplexing based onorthogonal codes. Similarly, DM RSs for antenna port 9 and antenna port10 may be located on REs of DM RS CDM group 2 through multiplexing basedon orthogonal codes.

FIG. 8 illustrates exemplary CSI-RS patterns defined for the LTE-Asystem. In FIG. 8, the positions of REs carrying CSI-RSs in an RBcarrying downlink data (an RB having 14 OFDM symbols in time by 12subcarriers in frequency in case of a normal CP) are marked. One of theCSI-RS patterns illustrated in FIGS. 8(a) to 8(e) is available for anydownlink subframe. CSI-RSs may be transmitted for eight antenna portssupported by the LTE-A system, antenna port 15 to antenna port 22.CSI-RSs for different antenna ports may be identified by their differentfrequency resources (subcarriers) and/or different time resources (OFDMsymbols). This means that the CSI-RSs may be multiplexed in FDM and/orTDM. CSI-RSs positioned in the same time-frequency resources fordifferent antenna ports may be identified by their different orthogonalcodes. That is, these DM RSs may be multiplexed in CDM. In theillustrated case of FIG. 8(a), CSI-RSs for antenna port 15 and antennaport 16 may be located on REs of CSI-RS CDM group 1 through multiplexingbased on orthogonal codes. CSI-RSs for antenna port 17 and antenna port18 may be located on REs of CSI-RS CDM group 2 through multiplexingbased on orthogonal codes. CSI-RSs for antenna port 19 and antenna port20 may be located on REs of CSI-RS CDM group 3 through multiplexingbased on orthogonal codes. CSI-RSs for antenna port 21 and antenna port22 may be located on REs of CSI-RS CDM group 4 through multiplexingbased on orthogonal codes. The same principle described with referenceto FIG. 8(a) is applicable to the CSI-RS patterns illustrated in FIGS.8(b) to 8(e).

The RS patterns illustrated in FIGS. 6, 7 and 8 are purely exemplary.Thus it should be clearly understood that various embodiments of thepresent invention are not limited to specific RS patterns. That is,various embodiments of the present invention can also be implemented inthe same manner when other RS patterns than those illustrated in FIGS.6, 7 and 8 are applied.

Cooperative Multi-Point (CoMP)

To satisfy enhanced system performance requirements for the 3GPP LTE-Asystem, CoMP transmission and reception technology known as co-MIMO,collaborative MIMO or network MIMO has been proposed. The CoMPtechnology can increase the performance of UEs located at a cell edgeand average sector throughput.

It is known that Inter-Cell Interference (ICI) generally degrades theperformance of a UE at a cell edge and average sector throughput in amulti-cellular environment with a frequency reuse factor of 1. To offeran appropriate throughput performance to a cell-edge UE in anenvironment constrained by interference, a simple ICI mitigationtechnique such as UE-specific power control-based Fractional FrequencyReuse (FFR) is used in the conventional LTE system. However, it may bepreferred to reduce the ICI or reuse the ICI as a desired signal for theUE, rather than to decrease the utilization of frequency resources percell. For this purpose, CoMP transmission techniques may be adopted.

Downlink CoMP schemes are classified largely into Joint Processing (JP),and Coordinated Scheduling/Beamforming (CS/CB).

According to the JP scheme, each point (eNB) of a CoMP unit may usedata. The CoMP unit refers to a set of eNBs used for a CoMP transmissionoperation. The JP scheme is further branched into joint transmission anddynamic cell selection.

Joint transmission is a technique of transmitting PDSCHs from aplurality of points (a part or the whole of a CoMP unit) at one time.That is, a plurality of transmission points may simultaneously transmitdata to a single UE. The joint transmission scheme can improve thequality of a received signal coherently or non-coherently and activelyeliminate interference to other UEs, as well.

Dynamic cell selection is a technique of transmitting a PDSCH from onepoint of a CoMP unit at one time. That is, one point of the CoMP unittransmits data to a single UE at a given time point, while the otherpoints of the CoMP unit do not transmit data to the UE at the timepoint. A point to transmit data to a UE may be dynamically selected.

Meanwhile, in the CS/CB scheme, a CoMP unit may perform cooperativebeamforming for data transmission to a single UE. While only a servingcell transmits data to the UE, user scheduling/beamforming may bedetermined through coordination among cells of the CoMP unit.

Uplink CoMP reception refers to uplink reception of a transmitted signalthrough coordination at a plurality of geographically separated points.Uplink CoMP schemes include Joint Reception (JR) and CS/CB.

In JR, a plurality of reception points receive a signal transmitted on aPUSCH. CS/CB is a technique in which while only one point receives aPUSCH, user scheduling/beamforming is determined through coordinationamong cells of a CoMP unit.

CSI-RS Configuration

In the LTE-A system supporting up to eight downlink Tx antennas, an eNBshould transmit CSI-RSs for all the antenna ports, as described before.Because transmission of CSI-RSs for up to eight Tx antenna ports inevery subframe leads to too much overhead, the CSI-RSs should betransmitted intermittently along the time axis to thereby reduce CSI-RSoverhead. Therefore, the CSI-RSs may be transmitted periodically atevery integer multiple of one subframe, or in a predeterminedtransmission pattern.

The CSI-RS transmission period or pattern of the CSI-RSs may beconfigured by the eNB. To measure the CSI-RSs, a UE should haveknowledge of a CSI-RS configuration that has been set for CSI-RS antennaports in its serving cell. The CSI-RS configuration may specify theindex of a downlink subframe carrying CSI-RSs, the time-frequencypositions of CSI-RS REs in the downlink subframe (e.g. a CSI-RS patternas illustrated in FIGS. 8(a) to 8(e)), a CSI-RS sequence (a sequenceused for CSI-RSs, generated pseudo-randomly based on a slot number, acell ID, a CP length, etc. according to a predetermined rule), etc. Thatis, a given eNB may use a plurality of CSI-RS configurations and mayindicate a CSI-RS configuration selected for use from among theplurality of CSI-RS configurations to a UE(UEs) in its cell.

To identify a CSI-RS for each antenna port, resources carrying theCSI-RS for the antenna port should be orthogonal to resources carryingCSI-RSs for other antenna ports. As described before with reference toFIG. 8, CSI-RSs for different antenna ports may be multiplexed in FDMusing orthogonal frequency resources, in TDM using orthogonal timeresources, and/or in CDM using orthogonal code resources.

When notifying UEs within the cell of CSI-RS information (i.e. a CSI-RSconfiguration), the eNB should first transmit to the UEs informationabout time-frequency resources (time information and frequencyinformation) to which a CSI-RS for each antenna port is mapped. To bemore specific, the time information may include the number of a subframecarrying CSI-RSs, a CSI-RS transmission period, a CSI-RS transmissionsubframe offset, and the number of an OFDM symbol carrying CSI-RS REsfor an antenna. The frequency information may include a frequencyspacing between CSI-RS REs for an antenna and a CSI-RS RE offset orshift value along the frequency axis.

FIG. 9 illustrates an exemplary periodic CSI-RS transmission. A CSI-RSmay be transmitted periodically at every integer multiple of onesubframe (e.g. in every 5, 10, 20, 40 or 80 subframes).

Referring to FIG. 9, one radio frame is divided into 10 subframes,subframe 0 to subframe 9. The eNB transmits a CSI-RS with a CSI-RStransmission period of 10 ms (i.e. in every 10 subframes) and a CSI-RStransmission offset of 3, by way of example. Different eNBs may havedifferent CSI-RS transmission offsets so that CSI-RSs transmitted from aplurality of cells are uniformly distributed in time. If a CSI-RS istransmitted every 10 ms, its CSI-RS transmission offset may be one of 0to 9. Likewise, if the CSI-RS is transmitted every 5 ms, the CSI-RStransmission offset may be one of 0 to 4. If the CSI-RS is transmittedevery 20 ms, the CSI-RS transmission offset may be one of 0 to 19. Ifthe CSI-RS is transmitted every 40 ms, the CSI-RS transmission offsetmay be one of 0 to 39. If the CSI-RS is transmitted every 80 ms, theCSI-RS transmission offset may be one of 0 to 79. A CSI-RS transmissionoffset indicates a subframe in which an eNB starts CSI-RS transmissionin every predetermined period. When the eNB signals a CSI-RStransmission period and offset to a UE, the UE may receive a CSI-RS fromthe eNB in subframes determined by the CSI-RS transmission period andoffset. The UE may measure a channel using the received CSI-RS and thusmay report such information as a Channel Quality Indicator CQI), a PMI,and/or a Rank Indicator (RI) to the eNB. Unless a CQI, a PMI and an RIare separately described herein, they may be collectively referred to asa CQI (or CSI). The above information related to the CSI-RS iscell-specific information common to UEs within the cell. A CSI-RStransmission period and offset may be set separately for each individualCSI-RS configuration. For example, CSI-RS transmission periods andoffsets may be separately set for a CSI-RS configuration for CSI-RSstransmitted with zero transmission power and a CSI-RS configuration forCSI-RSs transmitted with non-zero transmission power.

FIG. 10 illustrates an exemplary aperiodic CSI-RS transmission.Referring to FIG. 10, one radio frame is divided into 10 subframes,subframe 0 to subframe 9. Subframes carrying CSI-RSs may be indicated ina predetermined pattern. For instance, a CSI-RS transmission pattern maybe formed in units of 10 subframes and a 1-bit indicator may be set foreach subframe to indicate whether the subframe carries a CSI-RS. In theillustrated case of FIG. 10, the CSI-RS pattern tells that subframe 3and subframe 4 out of 10 subframes (i.e. subframe 0 to subframe 9) carryCSI-RSs. Such 1-bit indicators may be transmitted to a UE byhigher-layer signaling.

Various CSI-RS configurations are available as described above. Toenable a UE to receive CSI-RSs reliably for channel measurement, an eNBneeds to signal a CSI-RS configuration to the UE. Now a description willbe given below of embodiments of the present invention for signaling aCSI-RS configuration to a UE.

CSI-RS Configuration Signaling

The eNB may signal a CSI-RS configuration to the UE in two methods.

One of the methods is for the eNB to broadcast CSI-RS configurationinformation to UEs by Dynamic Broadcast CHannel (DBCH) signaling.

In the legacy LTE system, an eNB may transmit system information to UEson a Broadcast CHannel (BCH). If the system information is too much tobe transmitted on the BCH, the eNB may transmit the system informationin the same manner as downlink data transmission. Notably, the eNB maymask the CRC of a PDCCH associated with the system information by anSI-RNTI, instead of a particular UE ID. Thus, the system information istransmitted on a PDSCH like unicast data. All UEs within the cell maydecode the PDCCH using the SI-RNTI and thus acquire the systeminformation by decoding the PDSCH indicated by the PDCCH. Thisbroadcasting scheme may be referred to as DBCH signaling,distinguishably from general Physical BCH (PBCH) signaling.

Two types of system information are usually broadcast in the legacy LTEsystem. One type of system information is a Master Information Blok(MIB) transmitted on a PBCH and the other type of system information isa System Information Block (SIB) multiplexed with general unicast datain a PDSCH region. As the legacy LTE system defines SIB type 1 to SIBType 8 (SIB1 to SIB8) for system information transmission, a new SIBtype may be defined for CSI-RS configuration information which is newsystem information not defined as any conventional SIB type. Forexample, SIB9 or SIB10 may be defined and the eNB may transmit CSI-RSconfiguration information to UEs within its cell in SIB9 or SIB10 byDBCH signaling.

The other method for signaling CSI-RS configuration information is thatthe eNB transmits CSI-RS configuration information to each UE by RadioResource Control (RRC) signaling. That is, the CSI-RS configurationinformation may be provided to each UE within the cell by dedicated RRCsignaling. For example, while a UE is establishing a connection to theeNB during initial access or handover, the eNB may transmit the CSI-RSconfiguration information to the UE by RRC signaling. Alternatively oradditionally, the eNB may signal the CSI-RS configuration information tothe UE in an RRC signaling message requesting a channel state feedbackbased on CSI-RS measurement to the UE.

The foregoing two methods for signaling CSI-RS configurations and aCSI-RS configuration to be used for CSI feedback to a UE are applicableto the embodiments of the present invention.

CSI-RS Configuration Indication

The present invention provides a method for transmitting a CSI-RS in apredetermined subframe to a UE according to a CSI-RS configurationselected from among a plurality of available CSI-RS configurations by aneNB. According to the method, the eNB may signal the plurality of CSI-RSconfigurations to the UE and may also notify the UE of a CSI-RSconfiguration to be used for channel state measurement for CSI or CQIfeedback from among the plurality of CSI-RS configurations.

A description will be given of indication of a selected CSI-RSconfiguration and CSI-RSs to be used for channel measurement to a UE byan eNB according to an embodiment of the present invention.

FIG. 11 illustrates an example of using two CSI-RS configurations.Referring to FIG. 11, one radio frame is divided into 10 subframes,subframe 0 to subframe 9. For a first CSI-RS configuration (CSI-RS1), aCSI-RS transmission period is 10 ms and a CSI-RS transmission offset is3. For a second CSI-RS configuration (CSI-RS2), a CSI-RS transmissionperiod is 10 ms and a CSI-RS transmission offset is 4. The eNB maysignal the two CSI-RS configurations to the UE and notify the UE of aCSI-RS configuration to be used for CQI (or CSI) feedback.

Upon receipt of a CQI feedback request for a specific CSI-RSconfiguration from the eNB, the UE may measure a channel state usingonly CSI-RSs having the specific CSI-RS configuration. To be morespecific, a channel state is a function of a CSI-RS reception quality,the amount of noise/interference, and a correlation coefficient betweenthem. The CSI-RS reception quality may be measured using only theCSI-RSs having the specific CSI-RS configuration, and the amount ofnoise/interference and the correlation coefficient (e.g. an interferencecovariance matrix representing the direction of interference) may bemeasured in a subframe carrying the CSI-RSs or a predetermined subframe.For example, if the eNB requests a feedback for the first CSI-RSconfiguration to the UE, the UE may measure a reception quality usingCSI-RSs received in a fourth subframe, subframe 3 in a radio frame. Forthe UE to calculate the amount of noise/interference and the correlationcoefficient, the eNB may indicate an odd-numbered subframe to the UE.Alternatively or additionally, the eNB may confine the UE to a specificsingle subframe (e.g. subframe 3), for measuring the CSI-RS receptionquality and calculating the amount of noise/interference and thecorrelation coefficient.

For instance, the CSI-RS reception quality may be theSignal-to-Interference plus Noise Ratio (SINR) of the CSI-RSs, expressedas S/(I+N) (S is the strength of the received signal, I is the amount ofinterference, and N is the amount of noise). The strength of thereceived signal, S may be measured using CSI-RSs in a subframe carryingthe CSI-RSs as well as a signal for the UE. Since I and N vary accordingto the amount of interference from adjacent cells and the directions ofsignals from the adjacent cells, they may be measured using CRSstransmitted in a subframe designated for measuring S, or in a separatelydefined subframe.

The amount of noise/interference and the correlation coefficient may bemeasured on REs carrying CRSs or CSI-RSs in a subframe or on null REsdesignated to facilitate noise/interference measurement. To measurenoise/interference on CRS REs or CSI-RS REs, the UE may first recoverCRSs or CSI-RSs, acquire a noise and interference signal by subtractingthe recovered CRSs or CSI-RSs from a received signal, and thus calculatea statistical noise/interference value. A null RE is an empty RE withzero transmission power, carrying no signal. Null REs facilitatemeasurement of a signal transmitted from an eNB other than the eNB.While all of CRS REs, CSI-RS REs, and null REs may be used to calculatethe amount of noise/interference and the correlation coefficient, theeNB may designate specific REs for noise/interference measurement forthe UE, among the above REs. This is because appropriate REs need to beset for measurement at the UE depending on a neighbor cell transmits adata signal or a control signal on the REs. The neighbor cell maytransmit a data signal or a control signal on the REs according tosynchronization or non-synchronization between cells, a CRSconfiguration, and a CSI-RS configuration. Therefore, the eNB maydetermine the synchronization or non-synchronization between cells, theCRS configuration, and the CSI-RS configuration and designate REs formeasurement for the UE according to the determination. That is, the eNBmay indicate to the UE that the UE will measure noise/interference usingall or part of the CRS REs, CSI-RS REs and null REs.

For example, a plurality of CSI-RS configurations are available to theeNB. The eNB may indicate one or more CSI-RS configurations, and mayindicate to the UE a CSI-RS configuration selected for CQI feedback fromamong the CSI-RS configurations and the positions of null REs, for CSIfeedback. The CSI-RS configuration selected for CQI feedback may be aCSI-RS configuration with non-zero transmission power, relative to nullREs with zero transmission power. For example, the eNB may indicate oneCSI-RS configuration for channel measurement to the UE and the UE mayassume that CSI-RSs are transmitted with non-zero transmission power inthe CSI-RS configuration. Additionally, the eNB may indicate a CSI-RSconfiguration with zero transmission power (i.e. the positions of nullREs) to the UE and the UE may assume that the REs of the CSI-RSconfiguration have non-zero power. In other words, the eNB may notifythe UE of a CSI-RS configuration with non-zero transmission power and,in the presence of a CSI-RS configuration with zero transmission power,the eNB may indicate the positions of null REs in the CSI-RSconfiguration with zero transmission power to the UE.

As a modification example to the above-described CSI-RS configurationindication method, the eNB may signal a plurality of CSI-RSconfigurations to the UE and may also signal all or part of the CSI-RSconfigurations, selected for CQI feedback to the UE. Upon receipt of aCQI feedback for a plurality of CSI-RS configurations, the UE maymeasure CQIs using CSI-RSs corresponding to the CSI-RS configurationsand report the CQIs to the eNB.

To allow the UE to transmit the CQIs for the respective CSI-RSconfigurations, the eNB may predefine uplink resources for CQItransmission for each CSI-RS configuration and preliminarily provideinformation about the uplink resources to the UE by RRC signaling.

Additionally, the eNB may dynamically trigger CQI transmission for aCSI-RS configuration to the UE. The dynamic triggering of CQItransmission may be carried out through a PDCCH. The PDCCH may indicatea CSI-RS configuration for CQI measurement to the UE. Upon receipt ofthe PDCCH, the UE may feedback a CQI measurement result for the CSI-RSconfiguration indicated by the PDCCH to the eNB.

CSI-RSs may be set to be transmitted in different subframes or in thesame subframe in a plurality of CSI-RS configurations. If CSI-RSs havingdifferent CSI-RS configurations are transmitted in the same subframe, itis necessary to distinguish them. To identify the CSI-RSs havingdifferent CSI-RS configurations in the same subframe, one or more ofCSI-RS time resources, frequency resources, and code resources may bedifferent for them. For example, the positions of REs carrying CSI-RSsmay be different for different CSI-RS configurations in time or infrequency (for example, CSI-RSs with a CSI-RS configuration aretransmitted on REs illustrated in FIG. 8(a) in a subframe and CSI-RSswith another CSI-RS configuration are transmitted on REs illustrated inFIG. 8(b) in the same subframe). If CSI-RSs with different CSI-RSconfigurations are transmitted on the same RE, different CSI-RSscrambling codes may be applied to the CSI-RSs.

Application Examples of CSI-RS Configuration

The technical feature of the present invention that a plurality ofCSI-RS configurations are defined and a UE feeds back CQIs for theplurality of CSI-RS configurations can increase channel measurementperformance, when it is applied to a heterogeneous-network wirelesscommunication system, a Distributed Antenna System (DAS), a CoMP system,etc. However, the application examples of the present invention are notlimited thereto and it is clearly understood that a plurality of CSI-RSconfigurations can be defined and used in various multiple-antennasystems according to the principle of the present invention.

First of all, an application example of the present invention to aheterogeneous-network wireless communication system will be described. Aheterogeneous-network system may be a network where a macrocell and amicrocell are co-existent. The term ‘heterogeneous network’ may refer toa network where a macrocell and a microcell are co-located in spite ofthe same Radio Access Technology (RAT). A macrocell is a generic BShaving wide coverage and high transmission power in a wirelesscommunication system, whereas a microcell is a small-sized version ofthe macrocell such as a femtocell or a home eNB, capable of performingmost of the functions of the macrocell and independently operating.Within the heterogeneous network, a UE may be served directly from themacrocell (a macro UE) or directly from the microcell (a micro UE). Themicrocell may operate in a Closed Subscriber Group (CSG) manner or anOpen Subscriber Group (OSG) manner. The microcell serves only authorizedUEs in the former case and serves all UEs in the latter case. It mayoccur in the heterogeneous network that a downlink signal received fromthe macrocell at a UE near to the microcell, for example, at a UE nearto the microcell but not served by the microcell is severely interferedfrom a downlink signal from the microcell. Therefore, Inter-CellInterference Coordination (ICIC) is significant to the heterogeneousnetwork.

For efficient ICIC between heterogeneous cells in the heterogeneousnetwork environment, a plurality of CSI-RS configurations may be definedand channel quality may be measured according to the plurality of CSI-RSconfigurations. For example, if a limited time area is available to amicrocell, for example, the microcell is limited to even-numberedsubframes for signal transmission, and a macrocell uses differenttransmission power and beam directions in even-numbered and odd-numberedsubframes to reduce interference with the microcell, a macro UE mayexperience different channel quality in the even-numbered subframe fromin the odd-numbered subframe. Without taking into account the differentchannel environments of different subframes for the macro UE, a channelquality measured and reported by the macro UE may be different from thechannel quality of a real channel environment, thereby degrading overallnetwork performance. To avert this problem, different CSI-RSconfigurations may be applied to a plurality of different time areasunder different channel environments and thus the UE may measure andreport a CQI for each CSI-RS configuration using CSI-RSs receivedaccording to the plurality of CSI-RS configurations in accordance withthe foregoing various embodiments of the present invention.

Regarding an application example of the present invention to a DAS, aneNB may have a plurality of antennas at different positionssubstantially spaced from one another in the DAS. For example, giveneight antennas to an eNB, four antennas out of the eight antennas may beinstalled near to the eNB, two antennas out of the remaining fourantennas are installed at a remote place from the eNB and connected tothe eNB via an optical relay, and the other two antennas are installedat a remote place in the opposite direction from the eNB and connectedto the eNB via another optical relay. The eight antennas may be groupedinto three antenna groups having two, four and two antennas,respectively, according to their installation positions. In the DAS,different channel environments may be deployed according to thepositions of physical antennas. If CQIs are measured in the same mannerwith no regard for different channel environments, the real channelenvironments may not be measured correctly. To solve this problem, aneNB may allocate different CSI-RS configurations to a plurality ofantenna groups under different channel environments and may indicate oneor more CSI-RS configurations, and the eNB may provide a CSI-RSconfiguration selected for UE's CQI feedback from among the one or moreCSI-RS configurations and the positions of null REs to an individual UEby dedicated RRC signaling according to the various embodiments of thepresent invention. Or the eNB may indicate one or more CSI-RSconfigurations for UE's CQI feedback and the positions of null REs to anindividual UE by dedicated RRC signaling. The UE may measure and reporta CQI for the CSI-RS configuration set for CQI feedback using CSI-RSsreceived according to the CSI-RS configuration. In this manner, CQImeasurement and reporting may be carried out on a CSI-RS configurationbasis (i.e. on an antenna group basis). For this purpose, the number ofantennas for each CSI-RS configuration may be set independently.

Now a description will be given of an application example of the presentinvention to a CoMP system. The CoMP system transmits a signal throughcooperation of a plurality of cells to improve performance. CoMPtransmission/reception refers to communication between a UE and an eNB(an AP or a cell) through cooperation between two or more eNBs (APs orcells). The term ‘eNB’ is interchangeably used with ‘cell’, ‘AP’ or‘point’ in the CoMP system. CoMP schemes are largely classified intoCoMP-JP and CoMP-CS/CB. In CoMP-JP, CoMP eNBs simultaneously transmitdata to a UE at a given point of time and the UE combines the receivedsignals, thereby increasing reception performance. On the other hand, inCoMP-CS/CB, one eNB transmits data to a UE at a given point of time,while UE scheduling or beamforming is performed to minimize interferencefrom other eNBs.

For a reliable CoMP operation, the UE should measure CSI-RSs fromneighbor cells included in a CoMP unit as well as CSI-RSs from a servingcell and feed back measured channel information to the eNB. Therefore,the eNB needs to notify the UE of the CSI-RS configurations of theserving cell and the neighbor cells. According to the afore-describedembodiments of the present invention, the eNB may indicate to the UE aplurality of CSI-RS configurations as if these CSI-RS configurationswere for the eNB, and may also indicate to the UE a CSI-RS configurationfor channel information feedback selected from among the CSI-RSconfigurations.

On the assumption that a serving cell with A Tx antennas and a neighborcell with B Tx antennas cooperate for communication, the following threeCSI-RS configurations may be defined.

CSI-RS Configuration 1: the CSI-RS configuration of the serving cell(CSI-RSs for the A Tx antennas)

CSI-RS Configuration 2: the CSI-RS configuration of the neighbor cell(CSI-RSs for the B Tx antennas)

CSI-RS Configuration 3: the CSI-RS configuration of a virtual singlecell (CSI-RSs for the A+B Tx antennas)

The UE may feed back channel information as illustrated in Table 1according to the CSI-RS configurations indicated by the eNB.

TABLE 1 Configu- Configu- Configu- ration 1 ration 2 ration 3 CSIfeedback contents Case 1 On Off Off CSI feedback for Serving cell Case 2Off On Off CSI feedback for neighbor cell Case 3 On On Off Joint orseparate CSI feedbacks for serving cell and neighbor cell Case 4 Off OffOn CSI feedback for virtual single cell Case 5 On Off On CSI feedbackfor serving cell and CSI feedback for virtual single cell

When the above plurality of CSI-RS configurations are defined, the UEdoes not need to identify a cell that transmits CSI-RSs according toeach CSI-RS configuration. The UE has only to measure CSI-RSs receivedaccording to a CSI-RS configuration indicated by the eNB and feed backmeasured CSI to the eNB. Accordingly, an eNB may define a plurality ofCSI-RS configurations and indicate a CSI-RS configuration for CSIfeedback to a UE, and then the UE may measure and report channelinformation using CSI-RSs received according to the indicated CSI-RSconfiguration in the application example to the CoMP system according tothe present invention.

In the case where the plurality of CSI-RS configurations illustrated inTable 1 are defined, a serving eNB basically operates in Case 1. WhenCoMP information is needed, the serving eNB may acquire CSI required fora CoMP operation by configuring Case 2, Case 3 or Case 4 for a CoMPcandidate UE. The CSI required for CoMP may include channel informationbetween a neighbor cell and the CoMP candidate UE, channel informationbetween a serving cell and the CoMP candidate UE, and CoMP CSI for anassumed CoMP operation, and CSI for a virtual single cell with A+Bantennas. In each case, the UE operates in the following manner.

In Case 1, the UE may measure CSI-RSs received from the serving cellaccording to CSI-RS Configuration 1 and feed back CSI for the servingcell to the eNB. The CSI is same as CSI that the eNB receives from theUE in a non-CoMP environment.

In Case 2, the UE may measure CSI-RSs received from the neighbor cellaccording to CSI-RS Configuration 2 and feed back CSI for the neighborcell to the eNB. The UE regards the measured channel as one from theserving cell without any need to identify a cell that transmits thechannel. From the perspective of the UE, although only channels to bemeasured are different in Case 1 and Case 2, CSI may be generated in thesame manner in both cases.

In Case 3, the UE may measure CSI-RSs received from the serving cellaccording to CSI-RS Configuration 1 and CSI-RSs received from theneighbor cell according to CSI-RS Configuration 2 and generate CSIseparately for the serving cell and the neighbor cell. For the CSIgeneration, the UE may measure channels, regarding them as received fromthe serving cell without any need to identify actual cells that transmitthe channels. The UE may transmit the CSI generated according to CSI-RSConfigurations 1 and 2, together or separately on the downlink to theeNB.

Alternatively or additionally, the UE may generate CoMP CSI, assuming aspecific CoMP operation in Case 3. For example, on the assumption ofCoMP-JP, the UE may calculate a rank and a CQI that may be achieved fromjoint transmission, select a PMI from a joint-transmission codebook, andfeed back an RI, the PMI, and the CQI to the eNB.

In Case 4, the UE may measure CSI-RSs for a virtual single cell with A+Bantennas according to CSI-RS Configuration 3. To be more specific, theUE receives part of the CSI-RSs from the serving cell and the remainingCSI-RSs from the neighbor cell. For successful implementation of Case 4,each of the eNBs of the serving cell and the neighbor cell shouldtransmit CSI-RSs at the positions of CSI-RS REs of the virtual singlecell with the A+B antennas. For example, if the CSI-RSs of the singlecell with the A+B antennas are allocated to RE1 to RE(A+B), the servingcell should transmit CSI-RSs on RE1 to REA and the neighbor cell shouldtransmit CSI-RSs on RE(A+1) to RE(A+B). If the REs carrying the CSI-RSsfrom the serving cell and the neighbor cell satisfy the above condition,the operation is successful. Otherwise, an additional CSI-RStransmission may be needed.

In general, CSI-RSs are mapped to REs according to the number ofantennas in a tree structure as illustrated in FIG. 12. Referring toFIG. 12, 8Tx CSI-RS represents a group of REs to which CSI-RSs for eightTx antennas are mapped. 4Tx CSI-RS represents a group of REs to whichCSI-RSs for four Tx antennas are mapped. 2Tx CSI-RS represents a groupof REs to which CSI-RSs for two Tx antennas are mapped. As illustratedin FIG. 12, one 8 Tx CSI-RS RE group is the sum of two 4 Tx CSI-RS REgroups and one 4 Tx CSI-RS RE group is the sum of two 2 Tx CSI-RS REgroups. However, a 4 Tx CSI-RS RE group with RE #4 to RE #7 and another4 Tx CSI-RS RE group with RE #8 to RE #11 do not form an 8 Tx CSI-RS REgroup because of group misalignment in the tree structure. For example,if a serving cell with four Tx antennas uses RE #4 to #7 for CSI-RStransmission and a neighbor cell with four Tx antennas uses RE #8 to #11for CSI-RS transmission, the serving cell, the neighbor cell, or bothshould be able to map additional CSI-RSs so as to form one 8 Tx CSI-RSRE group for a CoMP candidate UE. That is, the serving cell may transmitnew 4Tx CSI-RSs on RE #12 to RE #15, the neighbor cell may transmit new4Tx CSI-RSs on RE #0 to RE #3, or each of the two cells may transmit new4Tx CSI-RSs, for example, the serving cell may transmit additional new4Tx CSI-RSs on RE #16 to RE #19 and the neighbor cell may transmitadditional new 4Tx CSI-RSs on RE #20 to RE #23. Therefore, the UE mayperceive the received CSI-RSs as 8Tx CSI-RSs.

Despite increased control signal overhead, transmission of additionalCSI-RSs to the CoMP candidate UE according to the characteristics of theCoMP candidate UE may increase network performance. In other words,although conventional CSI-RSs are designed to be universal so that allUEs within a cell can receive the CSI-RSs, the additional CSI-RSs for aCoMP operation are used only for the CoMP candidate UE in the aboveexample. Hence, CSI-RS design and transmission can be optimized for thepurpose. For example, the additional CSI-RSs may be precoded, takinginto account of a CoMP UE at a cell edge, so that they are steeredtoward the cell edge by beamforming. Or the additional CSI-RSs may beprecoded such that the spatial characteristics of channels measuredusing CSI-RSs received from the serving cell and the neighbor cell bythe UE are similar to the spatial characteristics of a virtual singlecell PMI codebook for the A+B Tx antennas. If precoding is applied toCSI-RSs, the eNB should additionally apply a precoder used for CSI-RSsas well as a precoder calculated based on CSI to actual data for theCoMP UE. That is, let transmission data, a precoding matrix acquiredusing CSI reported by the UE, and a precoding matrix used for CSI-RStransmission be denoted by x, W and W0, respectively. Then a signaltransmitted from the eNB is W0×W×x and the UE receives a signalY=H×W0×W×x+N from the eNB where N denotes noise.

In Case 4, the UE may generate and feed back CSI by measuring channels,on the assumption that the channels are from a serving cell with A+B Txantennas. For example, if A=B=4, the UE may generate an RI, a PMI and aCQI defined in an 8Tx single cell environment and feed back these valuesto the serving eNB.

In Case 5, the UE may measure CSI-RSs received from the serving cellwith A Tx antennas according to CSI-RS Configuration 1 and maysimultaneously measure CSI-RSs transmitted from the serving cell and theneighbor cell through with eight Tx antennas according to CSI-RSConfiguration 3. Therefore, the UE may generate non-CoMP CSI based onthe channel measurement using the CSI-RSs received according to CSI-RSConfiguration 1 and feedback the non-CoMP CSI to the eNB. In addition,the UE may generate CSI based on the channel measurement using theCSI-RSs received according to CSI-RS Configuration 3, considering themeasured channels to be channels from the serving cell with the A+B Txantennas and feedback the CSI to the eNB.

The above-described application examples of the present invention arepure exemplary, which should not be construed as limiting the presentinvention. That is, an eNB may set two or more CSI-RS configurations andnotify a UE of the CSI-RS configurations. The eNB may then command theUE to feed back CSI for all or part of the CSI-RS configurations. Hence,the UE may report the measurements of channel states for the CSI-RSconfigurations, together or separately on the uplink to the eNB. Thisprinciple of the present invention is applicable to various systemssupporting transmission through multiple antennas. CSI-RSs withdifferent CSI-RS configurations may be transmitted through the sameantenna group within the same cell by radiating antenna beams indifferent directions, through different antenna groups geographicallyapart from one another within the same cell, or through antennas ofdifferent cells.

FIG. 13 is a diagram illustrating a signal flow for a method fortransmitting CSI-RS configuration information according to an embodimentof the present invention. While an eNB and a UE are described in FIG. 13for illustrative purposes, the operation may take place between an eNBand a relay or between a relay and a UE in the same manner.

On or more CSI-RS configurations are available to an eNB. A CSI-RSconfiguration may include a configuration for time, frequency and/orcode resources allocated for transmission of CSI-RSs. For instance,CSI-RSs may be transmitted in one of the patterns (i.e. time-frequencypositions) illustrated in FIGS. 8(a) to 8(e) according to the CSI-RSconfiguration. The CSI-RS configuration may specify the positions of REsto which the CSI-RSs are mapped according to the number of antenna ports(e.g. 1, 2, 4 or 8) through which the CSI-RSs are transmitted.

One of the one or more CSI-RS configurations available to the eNB mayindicate the positions of REs carrying CSI-RSs for channel measurementat a UE, that is, the positions of REs carrying CSI-RSs with non-zerotransmission power. If there are CSI-RSs transmitted with zerotransmission power, for example, if a neighbor eNB transmits CSI-RSs,the one or more CSI-RS configurations available to the eNB may include aCSI-RS configuration indicating the positions of REs carrying CSI-RSswith zero transmission power. An operation of the eNB will first bedescribed below.

Referring to FIG. 13, the eNB may transmit information about one or moreCSI-RS configurations to the UE (S1310). The one or more CSI-RSconfigurations may include a CSI-RS configuration in which the UEassumes non-zero transmission power for CSI-RSs, that is, a CSI-RSconfiguration for CSI-RSs for use in channel measurement at the UE. Inaddition, the eNB may transmit to the UE information indicating a CSI-RSconfiguration in which the UE assumes zero transmission power forCSI-RSs, that is, a CSI-RS configuration indicating null REs as CSI-RSREs in step S1310.

The eNB may map CSI-RSs to REs in a downlink subframe according to theone or more CSI-RS configurations (S1320). The downlink subframe towhich the CSI-RSs are mapped may be configured according to acell-specific CSI-RS transmission period and a CSI-RS transmissionoffset. The CSI-RS transmission period and the CSI-RS transmissionoffset may be set separately for each CSI-RS configuration. For example,CSI-RS transmission periods and CSI-RS transmission offsets may be setdifferently for CSI-RSs for which the UE assumes non-zero transmissionpower and CSI-RSs for which the UE assumes zero transmission power.

The eNB may transmit the downlink subframe to the UE (S1330) and receiveCSI that is measured using the CSI-RSs from the UE (S1340).

Now a description will be given of an operation of the UE.

The UE may receive the information about the one or more CSI-RSconfigurations from the eNB (S1350). The one or more CSI-RSconfigurations may include a CSI-RS configuration in which the UEassumes non-zero transmission power for CSI-RSs, that is, a CSI-RSconfiguration for CSI-RSs for use in channel measurement at the UE. Inaddition, the eNB may transmit to the UE information indicating a CSI-RSconfiguration in which the UE assumes zero transmission power forCSI-RSs, that is, a CSI-RS configuration indicating null REs as CSI-RSREs in step S1350.

The UE may receive the downlink subframe to which CSI-RSs are mapped(S1360). A cell-specific CSI-RS transmission period and a CSI-RStransmission offset may be set cell-specifically or separately for eachCSI-RS configuration.

The UE measures a downlink channel using the received CSI-RSs andgenerates CSI (an RI a PMI, a CQI, etc.) based on the downlink channelmeasurement and the CSI-RS configuration indicating null REs (S1370).The UE may report the CSI to the eNB (S1380).

The afore-described embodiments of the present invention may beimplemented individually or two or more embodiments of the presentinvention may be implemented simultaneously in the method for providingCSI-RS configuration information described above with reference to FIG.13. Redundant descriptions are omitted for clarity.

FIG. 14 is a block diagram of an eNB apparatus and a UE apparatusaccording to an embodiment of the present invention.

Referring to FIG. 14, an eNB apparatus 1410 may include an Rx module1411, a Tx module 1412, a processor 1413, a memory 1414, and a pluralityof antennas 1415. The plurality of antennas 1415 support MIMOtransmission and reception. The Rx module 1411 may receive uplinksignals, data and information from UEs. The Tx module 1412 may transmitdownlink signals, data and information to UEs. The processor 1413 mayprovide overall control to the eNB apparatus 1410.

In accordance with an embodiment of the present invention, the eNBapparatus 1410 may be adapted to transmit CSI-RSs for transmissionthrough multiple antennas. The processor 1413 may transmit informationabout one or more CSI-RS configurations to a UE apparatus 1420 throughthe Tx module 1412. The one or more CSI-RS configurations may include aCSI-RS configuration indicating transmission of CSI-RSs with non-zerotransmission power. In addition, the processor 1413 may transmitinformation indicating a CSI-RS configuration indicating transmissionCSI-RSs with zero transmission power among the one or more CSI-RSconfigurations to the UE apparatus 1420 through the Tx module 1412. Theprocessor 1413 may map CSI-RSs to REs in a downlink subframe accordingto the one or more CSI-RS configurations. The processor 1413 maytransmit the downlink subframe to the UE apparatus 1420 through the Txmodule 1412.

Besides, the processor 1413 processes information received at the eNBapparatus 1410 and transmission information. The memory 1414 may storethe processes information for a predetermined time. The memory 1414 maybe replaced with a component such as a buffer (not shown).

The UE apparatus 1420 may include an Rx module 1421, a Tx module 1422, aprocessor 1423, a memory 1424, and a plurality of antennas 1425. Theplurality of antennas 1425 support MIMO transmission and reception. TheRx module 1421 may receive downlink signals, data and information froman eNB. The Tx module 1422 may transmit uplink signals, data andinformation to an eNB. The processor 1423 may provide overall control tothe UE apparatus 1420.

In accordance with an embodiment of the present invention, the UEapparatus 1420 may be adapted to transmit CSI using CSI-RSs receivedfrom an eNB supporting transmission through multiple antennas. Theprocessor 1423 may receive information about one or more CSI-RSconfigurations from the eNB apparatus 1410 through the Rx module 1421.The one or more CSI-RS configurations may include a CSI-RS configurationindicating transmission of CSI-RSs with non-zero transmission power. Inaddition, the processor 1423 may receive information indicating a CSI-RSconfiguration indicating transmission CSI-RSs with zero transmissionpower among the one or more CSI-RS configurations to the eNB apparatus1120 through the Rx module 1421. The processor 1423 may receive adownlink subframe in which CSI-RSs are mapped to REs according to theone or more CSI-RS configurations from the eNB apparatus 1420 throughthe Rx module 1421. The processor 1423 may measure CSI using the CSI-RSsand transmit the CSI measurement result to the eNB apparatus 1410through the Tx module 1422.

Besides, the processor 1423 processes information received at the UEapparatus 1420 and transmission information. The memory 1424 may storethe processes information for a predetermined time. The memory 1424 maybe replaced with a component such as a buffer (not shown).

The specific configurations of the above eNB and UE apparatuses may beimplemented such that the various embodiments of the present inventionare performed independently or two or more embodiments of the presentinvention are performed simultaneously. Redundant matters will not bedescribed herein for clarity.

The same description of the eNB apparatus 1410 is applicable to a relayas a downlink transmitter or an uplink receiver, and the samedescription of the UE apparatus 1420 is applicable to the relay as adownlink receiver or an uplink transmitter.

The embodiments according to the present invention can be implemented byvarious means, for example, hardware, firmware, software, or theircombination.

If the embodiment according to the present invention is implemented byhardware, the embodiment of the present invention can be implemented byone or more application specific integrated circuits (ASICs), digitalsignal processors (DSPs), digital signal processing devices (DSPDs),programmable logic devices (PLDs), field programmable gate arrays(FPGAs), processors, controllers, microcontrollers, microprocessors,etc.

If the embodiment according to the present invention is implemented byfirmware or software, the embodiment of the present invention may beimplemented by a type of a module, a procedure, or a function, whichperforms functions or operations described as above. A software code maybe stored in a memory unit and then may be driven by a processor. Thememory unit may be located inside or outside the processor to transmitand receive data to and from the processor through various means whichare well known.

Various embodiments have been described in the best mode for carryingout the invention. The detailed description of the exemplary embodimentsof the present invention has been given to enable those skilled in theart to implement and practice the invention. Although the invention hasbeen described with reference to the exemplary embodiments, thoseskilled in the art will appreciate that various modifications andvariations can be made in the present invention without departing fromthe spirit or scope of the invention described in the appended claims.For example, those skilled in the art may use each constructiondescribed in the above embodiments in combination with each other.Accordingly, the invention should not be limited to the specificembodiments described herein, but should be accorded the broadest scopeconsistent with the principles and novel features disclosed herein.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

Although the description of the above-described embodiments of thepresent invention is focused mainly on a 3GPP LTE group system, thepresent invention will not be limited only to the exemplary assumptionmade in the description of the present invention. Herein, theembodiments of the present invention may be used and applied in varioustypes of mobile communication systems having the MIMO technique appliedthereto, by using the same principle.

What is claimed is:
 1. A method for transmitting channel stateinformation (CSI) to a base station by a mobile station in a wirelesscommunication system, the method comprising: receiving, by the mobilestation, first information about one or more channel quality measurementresources and second information about one or more interferencemeasurement resources from a base station; receiving, by the mobilestation, reference signals based on the first information from the basestation; performing, by the mobile station, channel quality measurementbased on the reference signals; performing, by the mobile station,interference measurement based on the second information; generating, bythe mobile station, the CSI based on results of the channel qualitymeasurement and the interference measurement; and transmitting, by themobile station, the CSI to the base station, wherein the secondinformation about one or more interference measurement resourcesindicates positions of resource elements (REs) muted by the basestation.
 2. The method according to claim 1, wherein the firstinformation about one or more channel quality measurement resourcesindicates positions of REs for the reference signals.
 3. The methodaccording to claim 1, wherein the second information about one or moreinterference measurement resources indicates positions of REs wherereference signals of a neighbor base station are transmitted.
 4. Themethod according to claim 1, wherein the reference signals are channelstate information—reference signals (CSI-RSs).
 5. A method for receivingchannel state information (CSI) from a mobile station by a base stationin a wireless communication system, the method comprising: transmitting,by the base station to the mobile station, first information about oneor more channel quality measurement resources and second informationabout one or more interference measurement resources; transmitting, bythe base station to the mobile station, reference signals based on thefirst information; and receiving, by the base station from the mobilestation, the CSI generated based on results of channel qualitymeasurement and interference measurement, wherein the channel qualitymeasurement is performed based on the reference signals by the mobilestation, wherein the interference measurement is performed based on thesecond information by the mobile station, and wherein the secondinformation about one or more interference measurement resourcesindicate positions of resource elements (REs) muted by the base station.6. The method according to claim 5, wherein the first information aboutone or more channel quality measurement resources indicates positions ofREs for the reference signals.
 7. The method according to claim 5,wherein the second information about one or more interferencemeasurement resources indicates positions of REs where reference signalsof a neighbor base station are transmitted.
 8. The method according toclaim 5, wherein the reference signals are channel stateinformation—reference signals (CSI-RSs).
 9. A mobile station in awireless communication system, the mobile station comprising: areceiver; a transmitter; and a processor configured to: control thereceiver to receive first information about one or more channel qualitymeasurement resources and second information about one or moreinterference measurement resources from a base station, control thereceiver to receive reference signals based on the first informationfrom the base station, perform channel quality measurement based on thereference signals, perform interference measurement based on the secondinformation, generate channel state information (CSI) based on resultsof the channel quality measurement and the interference measurement, andcontrol the transmitter to transmit the CSI to the base station, whereinthe second information about one or more interference measurementresources indicates positions of resource elements (REs) muted bathebase station.
 10. The mobile station according to claim 9, wherein thefirst information about one or more channel quality measurementresources indicates positions of REs for the reference signals.
 11. Themobile station according to claim 9, wherein the second informationabout one or more interference measurement resources indicates positionsof REs where reference signals of a neighbor base station aretransmitted.
 12. The mobile station according to claim 9, wherein thereference signals are channel state information—reference signals(CSI-RSs).
 13. A base station in a wireless communication system, thebase station comprising: a receiver; a transmitter; and a processorconfigured to: control the transmitter to transmit, to a mobile station,first information about one or more channel quality measurementresources and second information about one or more interferencemeasurement resources, control the transmitter to transmit, to themobile station, reference signals based on the first information, andcontrol the receiver to receive, from the mobile station, channel stateinformation (CSI) generated based on results of channel qualitymeasurement and interference measurement, wherein the channel qualitymeasurement is performed based on the reference signals by the mobilestation, wherein the interference measurement is performed based on thesecond information by the mobile station, and wherein the secondinformation about one or more interference measurement resourcesindicates positions of muted by the base station.
 14. The base stationaccording to claim 13, wherein the first information about one or morechannel quality measurement resources indicates positions of REs for thereference signals.
 15. The base station according to claim 13, whereinthe second information about one or more interference measurementresources indicates positions of REs where reference signals of aneighbor base station are transmitted.
 16. The base station according toclaim 13, wherein the reference signals are channel stateinformation—reference signals (CSI-RSs).